Electrical piezometer apparatus



5 July 18, 1950 c. c. MINTER 2,515,785

ELECTRICAL PIEYZOMETER APPARATUS Filed D80. 4, 1947 ,.4 @v9-jif Z4 -LI15 ze EPE' EE- a I Heftige/EE ATTORNEY Patented .uiy 18, 1950ELECTRICAL PIEZOMETER APPARATUS Clarke C. Minter, Washington, D. C.,assignor to Minter Instrument Corporation, New York, N. Y., acorporation f New York Application December 4, 1947, Serial N o.'789,683

4 Claims. (Cl. 201-48) The present invention relates to pressuremeasuring devices and describes novel apparatus for quantitativelyconverting mechanical displacements due to pressure changes intoequivalent changes in the condition of an electrical circuit andconstitutes an improvement over the electrical piezometer shown andclaimed in my prior Patent No. 2,358,467, issued September 19, 1944.

The most common means for converting mechanical displacements intochanges in an electrical circuit is that in which the mechanicaldisplacement is made to actuate a movable contact sliding over asuitable resistance which is part of an electrical circuit. This type of`pickup is not very satisfactory owing to variable friction at thesliding contact.

In my above noted prior patent the use of Sliding contacts is eliminatedand I utilize the variation in thermal conductivity of a confined bodyof vgas (helium at 1 mm. pressure) as the volume of a spring-loadedbellows which is varied by external pressure to change the resistance ofa current-heated lament forming part of a Wheatstone bridge circuit. Thepresent invention employs the same basic principle, namely, the effectof pressure on the rate at which a current-heated lament loses heat in aconfined body of gas. |The improvement of the ,present inventionconsists in arranging the currentheated filaments in such a manner thatwhen the external pressure decreases, for example, one of the nlamentsincreases in resistance While the other decreases. l

The present invention together with the objects thereof may be bestunderstood as the specification proceeds when taken in conjunction withthe accompanying drawing wherein:

Fig. l is a cross-sectional View showing one form which thepresentinvention may take;

Fig. 2 is a cross-sectional View taken on the line III- II of Fig. 1;

Fig. 3 is a view similar to that of Fig. 1 but showing anothermodication which the present invention may take;

Fig. 4 is a sectional view taken `on the line IV-IV of Fig. 3, and

Fig. 5 lis a graph showing the variation in the thermal conductivity ofhelium with chan-gesV 'in pressure.

Referring now to the drawings .in detail, the device as shown in Fig. 1comprises .a cylinder 5, hermetically sealed at one end by a threadedlyengaging end plate 6 and having an inlet 1 at its opposite end, to whicha conduit or the like from a source of variable pressurais adapted tothreadedly engage a projecting bib 8. The endplate 6 has an expansibleand contractible bellows 5 hermetically sealed to its inner surfaceleaving the opposite at end I-l of this bellows free to move. Alsowelded or otherwise afxed to the end fplate 6 is a U-shaped strap orbracket I2 which, at its inner end or base of the U-shape, has securedthereto a second expansible and contractible bellows I3 which is thusdisposed within the outer bellows 9. It will also be noted that afurther U-shaped strap or bracket I4 is welded or otherwise secured tothe movable flat end lil of the outer bellows 9 and at its other end thestrap Ill is secured to the far end I5 of the bellows I3 opposite to theend to which the bracket I2 is connected. Also hermetically sealedthrough the end plate 6 are a plurality of leading-in conductors I6, IIand I8, with the conductors I1 and I8 being connected to and supportinga lament 20 Within the interior of the outer bellows 9 and theconductors I6 and I8 extending hermetically through the end of the innerbellows I3 and connected to a lilament 22 disposed interiorly of theinner bellows I3.

The entire unit as described, being carried by the end plate 6, isaccordingly assembled and secured to the end plate prior to insertion inthe cylinder 5. After assembly, the bellows 9 and I3, which are designedto have substantially equal free internal volume, so as to compensatefor external temperature changes, are then filled with a suitable gas ata pressure of about 1 mm. of mercury and having high thermalconductivity, such as hydrogen or helium, and sealed ofi at the tips 23.I prefer to employ helium as the filling gas because it is less likelyto diiuse through the walls of the bellows. Upon completion of theassembly of the various elements, the end plates 6 carrying suchelements, is then screwed into the cylinder 5 in an air-tight manner, byutilization of a suitable gasket 24. Although the complete electricalcircuit has not been vshown herein, the filaments 2li and 22 may beconnected by the leading-in conductors I6, II and IB to the identicalWheatstone bridge arrangement in the same manner as the filaments in myabove noted Patent No. 2,358,457, to quantitatively measure pressurevariations as therein described.

In the modication of Figs. 3 and 4, the lilaments 30 and 32 are mountedin cells or recesses 33 and 34 formed in the end plate 35. Also in thismodification the end plate 35 is of slightly larger diameter andthreadedly engages an annular collar 36 formed on the cylinder 31 with a,3 sealing gasket 38 positioned therebetween to make the connectionair-tight. A sealed closure 39 constitutes the outer wall of the cells33 and 34 as well as the outer wall of the end plate 35. In lieu of theinner bellows do being supported by a strip or bracket as in Fig. 1, itsend 42 is supported from the end plate 35 by a U-shaped hollow tubularmember 43 which forms a passageway from the cell 34 to the interior ofthe inner bellows 49. Also the end plate 35 is provided with apassageway @9 which establishes communication between the cell 33 andthe interior of the outer bellows 45. In all other respects themodcation of Figs. 3 and 4 is the same as that of Figs. 1 and 2, withthe strap 46 tying the mov able end 41 of the outer bellows 45 to theend 48 of the inner bellows 49, as previously -described relative toFigs. l and 2. Also in this latter modification the end plate 35 isprovided with four hermetically sealed leading-in conductors 49, 5l), 52and 53, with the leading-in conductors 5o and 52 being connectedtogether externally of the unit instead of internally, as is the commonlead I8 of Fig. l.

Fig. 5 shows how the thermal conductivity of helium varies with pressurewith the latter constituting the abscissaand absolute thermalconductivity constitutingthe ordinate. The line A shows the approximatethermal conductivity of air for comparison purposes while Vthe curve Bshows the variation of helium. It will thus be noted that if thepressure of the gas is low (1 mm'. of mercury or less), the thermalconductivity thereof varies very rapidly with pressure (density).

Referring again to Figs. 1 and 2 when external gas pressure entersthrough the inlet 1, it acts on the flat end I l! of bellows 9. Theresponse of the 4dat end I3 is controlled by the stiffness of the outerbellows 9 and the inner bellows I3 which are so chosen that when theexternal pressure exceeds 1 mm. l-Igabsolute, the flat end II) movestoward the end plate 5. Due' to the connection of the straps Il betweenthe flat bellows end II'I and the end I5 of'bellows I3, the flat end I5is also moved toward the end plate 5, thus expand-V ing the innerbellowsv I3 to the same degree' that the outer Abellows 9 is compressed.At the samev time the pressure (density) of the gas within the bellows 9and I3 is compressed and expandedA respectively. The compression of thegas within the free space vof bellows 9 increase its thermalconductivity, as can be seen by reference toV Fig."

5, with the result that the current heated iilament 23, which may beconnected to a Wheat'- stone bridge arrangement, will lose heat at afaster rate thus causing its temperature and electricalresistance todecrease in proportion to the increase in pressure 'exerted on the ilatend Il! of the bollows 9.

vThe simultaneous expansion of bellows I3', due to movement of its endplate I5 by theyU -shaped strap I4 causes a decrease in the gas pressurewithin the bellows I3 thereby decreasing the thermal conductivity of thegas. This in turn results in the heated filament 22, which may likewisebe connected to the Wheatstone bridge arrangement, losing Vheat at aslower rate and causing an increase in the temperature land Are,-sistance of the filament 22. Thus the iilaments 20 and 22 are inverselyresponsive to pressure changes exerted on the flat bellows end II) andby connecting these filaments in a Wheatstone bridge arrangement, asshown in my above noted prior patent, such pressure changes caribereadily indicated on a suitable meter, or the changes in the resistanceof the filaments utilized to automatically operate machinery or thelike.

When the external pressure is about 1 mm. Hg

the two bellows 9 and I3 are completely relaxed,

due to the internal gas pressure of the helium being so chosen that thetotal inward force exerted by the pressures within bellows 9 and I3 onthe ilat bellows end I0, is equal to the external force exerted on thisbellows end I9. This means that -in which P is the external pressureapplied to lbellows end I3, P9 is the pressure in bellows 9 Pis is thepressure in bellows I3 and A10 and A15 are the area of the respectivebellows end plates IIt and It.

lunit is responsive is controlled by the stiffness of the bellows 9 andI3 together with the spacing between the angular projection of theU-shaped bracket Il-i relative to the fixed end of the bellows I3, whenthe vbellows 9 and I3 are in relaxed condition, with-V the maximumdisplacement of the bellows ends I and I5 depending on the elastic limitof the bellows 9 and I3.

The modification of Figs. 3 and 4 operates in the identical mannerY asthat above described, the only difference being that the laments 39 'aand 32 oi' vthismodication are not within the confines of the'respectivebellows 139 and 45. Instead, the pressure changes within the bellows li5are transmitted tothe filament 3U through the passageway l and in asimilar manner, the

`pressure changes within the inner bellows 4I! A'provided whichvisexceedingly sensitive in its operation. v-Bythe utilization ofelectrically heated filaments subjected to changes in pressure of afluid having high thermal conductivity, the temperatureA and hence Vtheelectrical resistance of these laments is simultaneously variedinverselyproportional to changes in pressure of an external force. Thusby suitably connecting these ilaments ina measuring or a controlcircuit, the circuit is thus responsive to external pressure changeswhich can be accurately measured or employed to accurately controlmachinery andthe like. p

Although one specific embodiment of the present invention has been shownand described, it is to be understood that still further modicationsthereof may be made without departing from the spirit and scope of theappended claims.

I claim:

l. Apparatus for converting mechanical dis placements of a flexiblemember into equivalent changes in the condition of elements of anelectrical circuit comprising two bodies of helium two sensitive bellowshaving their fixed ends suitably attached to a base plate and their freepressure-responsive ends connected together, a conducting filamentmounted in each of said two bodies of helium gas, and means forconnecting said conducting filaments into an electrical circuit.

2. Apparatus for converting mechanical displacements of a ilexiblemember into equivalent changes in the condition of elements of anelectrical circuit, comprising two bodies of helium gas, each at apreselected density and independently confined in the free internalvolumes of two unequally sensitive bellows, the smaller being suitablymounted inside the larger with the fixed ends of both of said bellowssuitably attached to the same base and their free pressure-responsiveends connected together, a conducting lament mounted in each of the saidtwo bodies of gas, and means for connecting said conducting filamentsinto an electrical circuit.

3. Apparatus for converting mechanical displacements of a flexiblemember into equivalent changes in the condition of elements of anelectrical circuit comprising two bodies of helium gas, each at apreselected density and independently conned in the free internalvolumes of two unequally sensitive bellows, each of said bellowscommunicating through suitable conduits with two similar closed cells, aconducting filament mounted in each of said cells, and means forconnecting said filaments into an electrical circuit.

4. Apparatus for converting mechanical displacements of a flexiblemember into equivalent changes in the condition of elements of anelectrical circuit comprising two bodies of helium gas, each at apreselected density and independently conned in the free internalvolumes of two unequally sensitive bellows, the smaller being mountedinside the larger with the Xed ends of both of said bellows attached tothe same base plate and their free pressure-responsive ends connectedtogether, two similar closed cells, two passageways each of whichconnects the interior of one of said bellows with one of said closedcells, a conducting filament mounted in each of said cells, and meansfor connecting said conducting filament into an electrical circuit.

CLARKE C. MINTER.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,358,467 Minter Sept. 19, 19442,400,467 Ruge May 14, 1946

